Listtree.mcc: forward MUIA_ContextMenu to super class, not NListtree
This way if a user inherits from Listtree and overrides
MUIM_ContextMenuBuild/Choice those functions will be
called. If the attribute is forward to NListtree, the overriden
functions in Listtree will not be called, because context menu
even handler will fire on NListtree level.
Since users in their overriden functions have to use Listtree methods
(no direct data access), all the access can be proxied.
git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@50826 fb15a70f-31f2-0310-bbcc-cdcc74a49acc